Carburetor Adjustment - Single S/S Adjustable Needle Valve
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
Repeat for additional carbs.
Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
Note: These adjustments are best performed on the boat, where air and water temperatures are similar to normal operation. If performed in a tank, make sure you are ducting or blowing exhaust away from the engine as fumes will circulate back into the air intake and skew your results.
Carburetor Adjustment - H/S Adjustable Needle
Most modern engines are equipped with fixed high speed jets, therefore the following procedure is unnecessary.
This requires 2 people: a driver and a mechanic. If the H/S adjustment has been changed significantly to the point where the engine will not start, commence with the adjustment set at ¾ turn out clockwise from lightly seated – most engines will start at that setting.
With the motor(s) running at full throttle, with the proper size screwdriver or the high speed knob, slowly start turning one of the H/S needles counterclockwise until the engine starts to bog down. Note the position, then turn it back clockwise until the motor misses or threatens to stall. Again, note the position.
At that point, back that needle valve out until the setting is at the mid point of the two settings noted above. You can experiment with fine adjustments and pick the one that (to your ears) has the highest rpm.
Now, go to the other High Speed needle (for multicarb engines) and repeat that procedure.
For multicarb engines, lift the center lever adjustment off the high ridge (depends on model), keeping it lifted until the point is facing the engine, then lower it into its proper position. (When you turn that lever now, you're adjusting both High Speed needle valves at the same time.)